It is known from xe2x80x9cSystem Margin Enhancement with Raman Gain in Multi-Span WDM-Transmissionxe2x80x9d, Srivastava et al., Technical Digest OFC ""99 Friday, Feb. 26, 1999, for distributed Raman amplifiers to be used in order to improve the signal-to-noise ratio. In a Raman amplifier, at least one pump signal is preferably fed in in the opposite direction to the transmission direction, and amplifies the transmitted data signal on the basis of the Raman effect. Since broadband wavelength-division multiplexed signals are normally transmitted, a number of pump sources are provided in order to obtain a Raman gain profile which is as flat as possible, or some other desirable wavelength-dependent Raman gain profile. In previously used Raman pump sources, the pump power levels of the individual laser diodes are kept constant via constant injection currents. However, this type of control of the laser output power levels can lead to fluctuations due to temperature changes. Furthermore, aging of the laser diodes can lead to a decrease in the pump power level. Previous control systems using monitor diodes on the rear end face of the lasers are not sufficient to keep the pump power level sufficiently constant.
Furthermore, with the WDM transmission systems that are now in use, the level of individual signal elements in the WDM signal must not vary even when individual channels are connected or disconnected. In addition, the amplifiers must be matched to the characteristics of the path sections; in particular, to the fiber type. Furthermore, it should be possible to connect and disconnect transmission bands.
Further problems occur when a changing number of transmission bands are intended to be used. The transmission bands influence one another, owing to the Raman effect.
An object of the present invention is, therefore, to keep the pump power levels constant. Further object elements are to design the Raman amplifier for WDM signals (wavelength-division multiplexed signals) with different numbers of channels, and to allow matching to different transmission paths and operating conditions.
The Raman amplifier according to the present invention has the advantage that the pump power levels and pump wavelengths are constant. This is achieved by individual filtering and control of the individual pump sources. Polarization mixers (polarizers) ensure that the polarization is scrambled in order to achieve a gain level which is independent of the polarization of the WDM signal.
Since the Raman amplifier is used in WDM systems, the connection or disconnection of one or more transmission channels should not have any influence on the other channels. The Raman amplifier is, therefore, operated in the linear area.
A controller is advantageous which drives the pump lasers optimally for matching to different transmission sections and transmission characteristics.
Additional features and advantages of the present invention are described in, and will be apparent from, the following Detailed Description of the Invention and the Figures.